1. Field of the Invention
The present invention relates generally to the field of multistaged lean prevaporizing premixing fuel injectors, and more particularly to an improved low emissions combustion system for a gas turbine engine using such injectors.
2. Description of the Prior Art
In a gas turbine engine, inlet air is continuously compressed, mixed with fuel in an inflammable proportion, and then contacted with an ignition source to ignite the mixture which will then continue to burn. The heat energy thus released then flows in the combustion gases to a turbine where it is converted to rotary energy for driving equipment such as an electrical generator. The combustion gases are then exhausted to atmosphere after giving up some of their remaining heat in a recuperator to the incoming air provided from the compressor.
Quantities of air greatly in excess of stoichiometric amounts are normally compressed and utilized to keep the combustor liner cool and dilute the combustor exhaust gases so as to avoid damage to the turbine nozzle and blades. Generally, primary sections of the combustor are operated near stoichiometric conditions which produce combustor gas temperatures up to approximately four thousand (4,000) degrees Fahrenheit. Further along the combustor, secondary air is admitted which raises the air-fuel ratio and lowers the gas temperatures so that the gases exiting the combustor are in the range of two thousand (2,000) degrees Fahrenheit.
It is well established that NOx formation is thermodynamically favored at high temperatures. Since the NOx formation reaction is so highly temperature dependent, decreasing the peak combustion temperature can provide an effective means of reducing NOx emissions from gas turbine engines as can limiting the residence time of the combustion products in the combustion zone. Operating the combustion process in a very lean condition (i.e., high excess air) is one of the simplest ways of achieving lower temperatures and hence lower NOx emissions. Very lean ignition and combustion, however, inevitably result in incomplete combustion and the attendant emissions which result therefrom. In addition, combustion processes cannot be sustained at these extremely lean operating conditions.
Examples of prior lean premixed fuel injectors for gas turbine engines are shown in U.S. Pat. No. 5,894,720 to Willis et al. and U.S. Pat. No. 6,016,658 to Willis et al., both of which are assigned to the assignee of the present invention.
It has also been proposed to utilize a staged prevaporized premixed fuel injector for turbine engines as shown in U.S. Pat. No. 6,174,160 to Lee et al.
Another prior art injector 300 is shown schematically in FIG. 9. Inner and outer tubular walls 302 and 304 define an air supply passage 306 therebetween. Heated exhaust air from a microturbine recuperator 308 is provided to the air supply passage 306. A liquid fuel injection nozzle 310 is located centrally within the inner tubular wall 302 and receives liquid fuel through a liquid fuel supply passage 312 and atomization air through atomization air supply passage 314. Upstream and downstream air inlets 316 and 318, respectively, are provided through the inner tubular wall 302 upstream and downstream of the liquid fuel nozzle 310. An air distributor 320 extends from the liquid fuel nozzle 310 to the inner tubular wall 302 and has a plurality of air passages 322 therethrough. A plurality of swirling slots 324 provide further communication of air from air supply passage 306 to the interior of inner tubular wall 302. The injector 300 has an outlet end 326 communicated with a combustion chamber 328 of the microturbine.
Thus, lean premixed combustion, both for liquid and gaseous fuels, has gained more popularity in order to achieve low NOx emissions in advanced combustion systems. On the other hand, due to fuel flexibility requirements, combustors are required to be more tolerant to fuel composition changes, which lean premixed combustors are sensitive to. Lean prevaporized premixed injector systems coupled with fuel composition changes are more subject to problems such as combustion instability, flashback and auto ignition.